This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
In Fig. 20 are shown the plan (a) and elevation (d) of a straight stairway with a 6-inch cylinder. The first requirement in planning this stairway is to fix the number and height of risers. The story rod A indicates the height of the story to be 9 feet 4 inches, or 112 inches, which, divided by 14, the number of risers, gives 8 inches as the height of each riser. The total run cd is 10 feet 1 inch, less 4 inches for the depth of the cylinder, which leaves 9 feet 9 inches to be divided into 13 treads, giving 9 inches as the width of each tread. As the floor at the landing makes one of the steps, there is one tread less than there are risers. The width of the stairway, shown in (a) as 2 feet 9 inches, is measured from the face of the plaster to the face of the front stringer, while the width of well hole includes the width of stairs, the diameter of the cylinder (in this case 6 inches), plus 1 inch for the thickness of the facia, and 1 inch for lath and plaster - altogether 3 feet 5 inches; and if the outside wall be constructed of brick, 1 inch more for thickness of furring will be required. At the landing of a straight flight, there must be at least as much room between the cylinder and the partition as the width of the stairway, preferably as much more as possible. In this case 3 feet is the width used in the platform, the 3 inches extra being allowed for convenience in moving furniture, etc.
The headroom of the stairway shown in Fig. 20 is 7 feet 2 inches from the top of the second step to the ceiling, and the point where this headroom can be secured governs the length of the well hole - as figured on the plan, 9 feet 4 inches. It is not necessary to make a drawing of the elevation in order to find the length of the well hole for the required headroom, as this can be calculated from the plan, by counting 12 risers down. If each riser is 8 inches, the 12 risers make 96 inches. From 96 inches subtract 10 inches, the depth of the floor, giving 7 feet 2 inches as the headroom. The 12 treads, of 9 inches each, make 9 feet, to which must be added 3 inches for the depth of the cylinder, and 1 inch for its thickness, making the total length of well hole 9 feet 4 inches.